Testosterone (T) is a primary androgenic hormone produced predominantly in the interstitial cells of the testes and is responsible for normal growth, development and maintenance of male sex organs and secondary sex characteristics (e.g., deepening voice, muscular development, facial hair, etc.). Throughout adult life, testosterone is necessary for proper functioning of the testes and its accessory structures, prostate and seminal vesicle; for sense of well-being; and for maintenance of libido, erectile potency, muscle mass, and bone health. Testosterone deficiency is insufficient secretion of T characterized by low serum T concentrations and can give rise to medical conditions (e.g., hypogonadism) in males. Symptoms associated with male hypogonadism include impotence and decreased sexual desire, fatigue and loss of energy, low mood and depressive symptoms, regression of secondary sexual characteristics, decreased muscle mass, and increased fat mass. Furthermore, hypogonadism in men is a risk factor for anemia, osteoporosis, metabolic syndrome, type II diabetes and cardiovascular disease.
Circulating free testosterone (FT) levels have been used widely in the diagnosis and treatment of hypogonadism in men. Testosterone is the second most frequently ordered endocrine test. In 2012, nearly 4 million free testosterone tests were performed in the USA alone. A number of direct and indirect methods —equilibrium dialysis, ultrafiltration, tracer analog methods, and calculations based on homogenous sex-hormone binding globulin (SHBG) and testosterone (SHBG:T) binding equations —have been developed for the determination of FT levels. Due to experimental complexities in FT measurements, the Endocrine Society expert panel has recommended the use of calculated FT (cFT) as an appropriate approach for estimating FT. Expert panels have expressed concern about the accuracy and methodological complexity of the available assays for FT (Rosner et al 2007,Sodergard et al 1982,Vermeulen et al 1971).